In forestry a critical statistic is the increase in girth of a tree trunk with time. This girth measurement must be taken periodically in order to determine the action of fertilizers, stand treatment, climate, and other factors. Since a tree grows fairly slowly, it is necessary that the measurement be done with considerable accuracy.
The simplest procedure is to take a flexible rule and wind it around a tree trunk at a given level above the ground, and then compare the measurement thus taken with the previous such measurement. Such a procedure is haphazard at best, since the trunk girth can vary somewhat in a range of a few inches of height. In addition it places on the person collecting the statistics the need to keep track of the previous measurement for that exact tree so as to be able to calculate the change in girth, which differential is the only real number of interest.
In another known system a flexible but inextensible tape measure is formed with a row of holes. It is wrapped around a tree trunk so that its end overlap, and a coil spring is hooked over the lapping ends between two holes of the tape to keep it snug on the tree. This tape is unlikely to move vertically much so that it will give a fairly accurate reading. Nonetheless it still leaves to the person taking the measurements the responsibility of keeping track of the previous measurement.
Similarly it is frequently necessary to determine the change in spacing between sides of a crack in wall or to determine if building parts are shifting relative to each other over time. Such a measurement is normally very difficult to make accurately.